JPH01156456A - Method for hot-working titanium ingot - Google Patents

Abstract

PURPOSE:To carry out the recrystallization refining of a structure in the surface layer of an ingot and to manufacture a Ti product excellent in surface characteristics by previously applying working strain to the surface layer of an ingot and then applying recrystallization to the above at the time of hot- working an ingot of pure Ti or Ti alloy. CONSTITUTION:At the time of subjecting an ingot of pure Ti or Ti alloy to hot working and cold working to manufacture the product of expanded material, such as plate stock, bar stock, wire rod, tube stock, and shape, working, such as forging and rolling, is previously applied to the ingot to apply working strain to the surface. Subsequently, heating is applied to the above up to a temp. of the recrystallization temp. of Ti or Ti alloy or above to subject the surface layer to recrystallization refining to >=at least 2mm thickness, followed by hot working, such as hot rolling and hot extrusion. By this method, surface ruggedness due to coarse crystalline grains at the ingot surface and further the occurrence of cracks and surface flaws in stripes can be prevented and, as a result, the expanded material product of Ti or Ti alloy excellent in surface characteristics can be manufactured.

Description

[Detailed description of the invention] [Field of industrial application] The present invention involves hot working a pure titanium or titanium alloy ingot cast into a slab or billet shape,
Or roughly cold-processed into plates, bars, wires, pipes, etc.
The present invention relates to a method for manufacturing wrought titanium products having good surface properties by preventing the occurrence of surface flaws due to the coarse cast structure of an ingot when manufacturing wrought titanium products such as shapes.

[Prior art] Pure titanium and titanium alloy wrought products generally have a VA
A cylindrical ingot melted in R (consumable electrode type vacuum arc melting furnace) is formed into a slab or billet by forging or blooming, and this is subjected to hot processing such as hot rolling or hot extrusion. Alternatively, it is manufactured by further cold working. However, VAR melting requires two melting steps to adjust the components, and since it is a consumable electrode type, there are problems such as the ability to remelt titanium scrap as a raw material for melting. Therefore, in recent years, EBR (electron beam melting) and VF
Development of new titanium melting methods such as R (vacuum plasma melting) is progressing. The feature of these new melting methods is that ingots with any cross-sectional shape can be manufactured, and by casting ingots in slab or billet shape, the so-called breakdown process such as conventional forging or blooming rolling can be omitted. is possible.

[Problems to be solved by the invention] However, when the ingot is directly subjected to hot rolling or hot extrusion without forging or blooming, the coarse grains of the ingot and each crystal are Due to the anisotropy of grain deformation, visible irregularities occur on the surface, which become cracks and streak-like surface defects as the processing rate increases. When turning these hot-processed materials into products, surface care is required, which causes a significant decrease in yield. For example, edge cracks occur during hot rolling of plates, and the edge cracks must be removed by edge trimming or the like when used as a hot rolled product or as a material for cold rolling. In addition, streak-like defects occur on the surface of hot-rolled wire materials and hot-extruded rod materials.
Care such as pickling and grinding is required.

An object of the present invention is to prevent surface flaws on titanium products caused by such coarse crystal grains of ingots, and to produce titanium products with good surface properties.

[Means and effects for solving the problem] Therefore, the present inventors investigated a means to prevent the occurrence of surface flaws by recrystallizing the structure of the surface layer of the ingot. By applying processing strain to at least the surface layer of the ingot by forging or rolling, and then heating it above the recrystallization temperature to recrystallize at least the surface layer of the ingot, surface flaws can be prevented during hot working. We have found that it is possible to reduce the The thickness of the recrystallized layer at this time is preferably 2 mm or more,
The present invention was achieved by confirming that there is almost no relation to the method of applying processing strain, the ingot size, or the recrystallized grain size.

Since the present invention is based on the premise of omitting forging and blooming, an ingot (slab or billet shape) of pure titanium or various titanium alloys was used as the material. The surface layer of these materials is processed by shot blasting, forging, roll reduction, etc., then heated, and hot processed (plate rolling, wire rolling, hot extrusion, etc.) to determine the recrystallized state of the surface layer and the surface. When we investigated the occurrence of defects, we found that the conditions for the occurrence of surface defects to be on the same level or better than when hot-processing forged or bloomed materials are the composition of the material, surface processing method, and recrystallization. Regardless of particle size,
It was found that the thickness of the recrystallized layer from the surface layer is preferably 2 mm or more. However, the material temperature during ingot processing needs to be in a temperature range where residual strain occurs on the surface due to processing, and is generally below the recrystallization temperature.

[Example] Example-1 Pure titanium JIS Class 2 and titanium alloy (Ti-5A1-2.
5 Sn) The side of the ingot is reduced by 1 by the amount of width reduction on one side.
Forging of mm, 2mm, 3mm and 5mm was performed.

Pure titanium was forged at room temperature (approximately 30° C.), and titanium alloys were forged at room temperature and after being heated to approximately 200° C. in an oil bath. The slab was made of pure titanium at a heating temperature of 860°C and a reduction rate of 9.
8%, titanium alloy heating temperature 1150℃, reduction rate 96%
Each was hot rolled.

For comparison, an as-cast slab and a hot-forged slab were similarly hot-rolled. Table 1 shows the results of investigating the thickness of the recrystallized layer on the surface of the slab and the edge cracking of the hot rolled material. It can be seen that by forging and recrystallizing the side surfaces of the slab before hot rolling, the occurrence of edge cracks in the hot rolled material can be reduced. And with pure titanium, by creating a recrystallized layer with a thickness of 2 mm or more on the surface,
A product with a surface quality without any problems was obtained.

In addition, with titanium alloy, a recrystallized layer with a thickness of 2 mm or more is generated on the surface, so the occurrence of edge cracking is the same as that of conventional materials, and the yield loss due to trimming after hot rolling can be suppressed to the same level as conventional materials. was completed.

Table 1 Example-2 Pure titanium JTS cast into a square billet with a length of 50m
The surface of the 2nd type and titanium alloy (Tl-6Al-4V) twin got is
Forging of 5 mm and 5 mm was performed. Pure titanium was forged at room temperature (approximately 30° C.), and titanium alloys were forged at room temperature and after being heated to approximately 200° C. in an oil bath. The billet was hot wire-rolled at a heating temperature of 820°C and an area reduction rate of 98% for pure titanium, and at a heating temperature of 1150°C and an area reduction rate of 98% for the titanium alloy. For comparison, an as-cast billet and a hot blooming billet were similarly hot rolled into wire rods. Regarding these, the thickness of the recrystallized layer on the billet surface layer,
Table 2 shows the results of investigating the depth of surface flaws in hot rolled materials. It can be seen that by forging and recrystallizing the billet surface before hot rolling, the occurrence of surface flaws in the hot rolled material is reduced. Furthermore, by producing a recrystallized layer with a thickness of 2 a+m or more, a product with a surface quality that was completely acceptable was obtained.

Table 2 Example-3 Pure titanium JIS cast into a round billet with a diameter of 170 mm
2f! The surface of the titanium alloy (Ti-6ALl-4V) twingots was rolled at room temperature (approximately 30°C) with rolling loads of 1 ton, 3 ton, 5 ton, and 8 ton in the circumferential direction using rolls with an outer diameter of 100 mm. . For pure titanium, the billet was heated at a heating temperature of 920°C and an area reduction rate of 98.
% and titanium alloys were hot extruded at a heating temperature of 960 t and an area reduction rate of 84%. For comparison, an as-cast billet and a billet obtained by hot blooming were also hot extruded. Regarding these, the thickness of the recrystallized layer on the billet surface layer,
Table 3 shows the results of investigating the depth of surface flaws in hot extruded materials.

It can be seen that by performing rolling to recrystallize the billet surface before hot extrusion, the occurrence of surface flaws in the hot extruded material is reduced. Furthermore, by producing a recrystallized layer with a thickness of 2 mm or more, a product with a surface quality that had no problems at all was obtained.

Table 3 [Effects of the Invention] As described above, according to the present invention, it is possible to omit the forging or blooming rolling that was conventionally necessary to refine the structure, and to produce an ingot of pure titanium or titanium alloy having a coarse structure. It is possible to obtain a product with good surface properties by directly subjecting it to hot processing, and the benefits of skipping the process are significant.

Claims (1)

[Claims] Thermal processing of titanium ingots is characterized by applying processing strain to the surface layer of a pure titanium or titanium alloy ingot, then heating it above the recrystallization temperature to recrystallize the structure of the surface layer, followed by hot working. Inter-processing method.